Cloning of the GABA(B) Receptor Subunits B1 and B2 and their Expression in the Central Nervous System of the Adult Sea Lamprey

In vertebrates, γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS) acting through ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. The GABA(B) receptor produces a slow inhibition since it activates second messenger systems through the binding and activation of guanine nucleotide-binding proteins [G-protein-coupled receptors (GPCRs)]. Lampreys are a key reference to understand molecular evolution in vertebrates. The importance of the GABA(B) receptor for the modulation of the circuits controlling locomotion and other behaviors has been shown in pharmacological/physiological studies in lampreys. However, there is no data about the sequence of the GABA(B) subunits or their expression in the CNS of lampreys. Our aim was to identify the sea lamprey GABA(B1) and GABA(B2) transcripts and study their expression in the CNS of adults. We cloned two partial sequences corresponding to the GABA(B1) and GABA(B2) cDNAs of the sea lamprey as confirmed by sequence analysis and comparison with known GABA(B) sequences of other vertebrates. In phylogenetic analyses, the sea lamprey GABA(B) sequences clustered together with GABA(B)s sequences of vertebrates and emerged as an outgroup to all gnathostome sequences. We observed a broad and overlapping expression of both transcripts in the entire CNS. Expression was mainly observed in neuronal somas of the periventricular regions including the identified reticulospinal cells. No expression was observed in identifiable fibers. Comparison of our results with those reported in other vertebrates indicates that a broad and overlapping expression of the GABA(B) subunits in the CNS is a conserved character shared by agnathans and gnathostomes.